The choice of the building site for the Basilica of St Peter in Rome was a long, laborious
process. The pre-existent basilica, dating from the reign of Constantine, had suffered from
severe problems of stability due to the fact that a fair portion of it rested on sand and gravel
of recent origin, having been deposited there by the periodic flooding of the Tiber – hardly
the sort of terrain with geo-technical characteristics one looks for in a construction site. The
new Christian basilica was wisely located on Pliocene era compacted clays, the best footings
available in the Eternal City. Other attempts made on alluvial terrain ran afoul of enormous
problems related to stability; even Bernini’s campanile was afflicted by cracks and partial
collapse, and finally razed in 1629. The problems, then, are ones of statics which have
nothing to do with the damage suffered by the monuments of Imperial Rome ranging from
the column known as the Colonna Antonina to Flavius’ Amphitheatre caused by earthquakes
unleashed in the Apennines – which damage for that matter was increased by the flooding
mentioned above.
Equally difficult was the choice of the ornamental materials for Rome’s new basilica, due to
geological conditions as well as to the particular historical context when the mother of all
churches was being built. Here we must go back to the XVI Century with Rome just having
been sacked by German mercenaries, the Lanzichenecchi, when the will to renewal was
stronger than ever. It is for this reason the Michelangelo was called upon at St Peter’s and that
Bernini had been granted full license to express his markedly naturalistic bent. Still, valuable
building materials plundered from the monuments of Classical Rome soon were exhausted
so that it was necessary to hunt up a fresh supply of stone nearby (so beautiful as to deserve
the name “marble”). The times were no longer suited for doing as the Caesars of Ancient
Rome had done and have unsullied white marble brought in from Carrara or Paros, or other
sumptuous building stone brought in from all over the Mediterranean. It is no mere
happenstance that for the most part ornamental materials used in Rome Baroque construction
came from no more than 60-70 kilometres away.
Thus it was that between 1627 and 1700 many old quarries were sought out and reactivated,
so that even today it is possible to see traces of these operations in old sites in the Sabine Hills
region just north of Rome. Still lying in a quarry near the town of Cottanello is a weathered
column which engineers of the time did not manage to transport to Rome. Cottanello
“marble” was just the answer to changed demands, being beautiful in appearance – deep red
fully veined in white –, close at hand and cheap, even though in reality it was not a true
marble (at least in a geological sense).
Cottanello “marble” is in reality a marl limestone (which in Italy locally takes the name
Scaglia) and was deposited over a vast sea basin some tens of millions of years ago in the
Umbro-Sabine region of central Italy. It a beautiful red in colour continuously veined in ever
changing patterns of pure white calcite, making its appearance is anything but homogenous.
The rock’s particular complexion is not, however, original in the sense that it is due not to
sedimentary but to tectonic processes, meaning the deformations it has undergone as being
a part of the Apennines, a mountain chain formed as a result of enormous compressive stress
that caused the European and African plates to collide for millions of years in the
Mediterranean. The calcite veins and such are the result of complex deformational processes
which have substantially changed the size and “form” of the original rock. In particular, a
major strike-slip fault running north-south the length of central Italy (the faglia sabina) is
responsible for the further increase in complexity of the pattern formed by the white of the
calcite and the red of the limestone. Whereas this fault may yet be in motion today, it is
certain that its most active period was around 5 million years ago when man had yet to make
his appearance and the landscape was quite different from today.
The Cottanello quarry is located precisely within the limited area where the faglia sabina
runs, with the material quarried there faithfully recording its greater or lesser proximity to the
principal sheer zone. The columns of Sant’Andrea at the Quirinale together with those of St
Peter’s Basilica were the first to be fashioned using marble from the quarry and therefore
have especially rich, complex veining – which makes the marble more valuable – having been
quarried in the heart of the most deformed fault location. The columns of Sant’Agnese in Agone
and the facing of Pope Alexander VII’s tomb in St Peter’s were instead fashioned in later
times and were therefore quarried in areas farther from the fault, being less deformed and
poorer in calcite veins. As one leaves the fault area the geological features change. Anyone can ‘visit’ the Cottanello quarry at a distance simply by taking a tour through the
extraordinary places just mentioned. The variegated appearance of the columns at St Peter’s
thus reflect not just the difficult times when the Basilica was under construction, but the
tormented geological history of the entire central Apennine region as well, with the “marble”
of Cottanello being an exemplary summation.